1. Field of the Invention
This invention relates to a video signal recording/reproducing apparatus for recording television signals in a recording media by compressing multiple fields of the television signal as a unit which is hereinafter referred to as multiple field unit, and reproducing the recorded television signals in normal play mode, high-speed search mode or slow play mode.
2. Description of the Prior Art
To form video recording/reproducing apparatuses (e.g. VTRs, video disc players) for recording and reproducing video signals, the bit rate reduction technology (or high efficiency coding technology) has been used to reduce the data quantity of the original video signal for a long time video recording. An example of the bit rate reduction technology is intra-frame processing. In the intra-frame processing, a two field unit of the video signal is compressed, that is, every frame of the video signal. The video signal in a television signal form is an interlaced signal, two fields of which constitutes one frame. By the intra-frame processing, the video signal is compressed to a non-interlaced form. The non-interlaced signal has the line-to-line distance (between scan lines) reduced to one-half of that for the interlaced signal. Therefore, when a still picture is displayed, the correlation between lines is high and the compression is done efficiently. Even when the displayed image is a moving picture, compression can be done with high efficiency by performing motion compensation between two fields. The high efficiency in the compression mentioned above is not unique to the intra-frame processing, but in general, the compression efficiency will be higher when the processing unit of video signals to be processed become larger so long as there is no scene change in the video signal.
However, in conventional video signal recording/reproducing apparatuses, if compression in multiple field units is applied, expansion in multiple field units is necessary in reproduction. Therefore, there is a problem, as described below, in trick plays, such as slow play and high-speed search.
With reference to processing in two field units (hereinafter referred to as intra-frame processing), this problem will be described. Let us first consider a field array which is reproduced and outputted for slow play. When the video signal recording/reproducing apparatus is performing compression and expansion in two field units, the video signal can be reproduced only in two field units at all times. Consequently, in slow play, the same frame is output repeatedly for a specified number of times corresponding to the reproducing speed at that time. With regard to the order of arrangement of fields for a period in which the same frame is output repeatedly, if the field that occurs ahead in time of the two fields constituting one frame is named as the former field and the field that occurs behind in time is denominated as the latter field, the former and latter fields are displayed alternately on the screen in the following order: former field--latter field--former field--latter field, and so on. For an array of a former field--latter field, two fields having between them a forward time difference which corresponds to one field and which is the same as when visual images were recorded, are displayed with a time difference placed in the forward direction. For an array of a latter field--former field, two fields having between them a reverse time difference corresponding to one field are displayed with a time difference placed in the forward direction. In a picture reproduced in the forward direction, a time difference in the reverse direction causes a flicker and severely deteriorates the picture quality of the moving picture.
A case of high-speed search will next be described. As for the field array of an output video signal, because the operation is a high-speed search, intermittent frames are arranged with a certain number of frames of video signal thinned out according to the reproducing speed. In a high-speed search in the forward direction, the order of arrangement of the fields is not reversed as in a slow play in the forward direction mentioned above. However, though the time difference between two fields constituting one frame as one processing unit corresponds to a period of one field when these fields are recorded the succeeding fields which are reproduced just after on the screen are those which constitute a processing unit after passage of a specified number of frames decided according to the reproducing speed. Therefore, in a high-speed search, there are mixed field arrays having only a time difference corresponding to one field and field arrays having a greater time difference due to the thinning out of frames for high speed search. As has been described, the presence of the arrays having irregular time differences in reproduced pictures is doubtless a big cause of picture quality deterioration in a moving picture.
From the foregoing description, it will be understood that in conventional video signal recording/reproducing apparatuses, in trick plays, if the time difference between fields within one processing unit differs from the time difference between processing units, and particularly in a slow play at normal speed or less, there are intermingled dissimilar time differences in the forward and reverse directions, thus substantially deteriorating the smoothness of the motion of the displayed pictures.